I agree with the evenflo chase or Graco Cargo (as you already have the comfortsport). This site shows you the top harness heights on each of the harnessed boosters, (and for most U.S. seats actually).http://www.carseatdata.org/csnumbers.html

The comfortsport has really low harness slots (I helieve the lowest of them all) and unfortunately I don't know of many children that are able to stay in it until the are anywhere near ready for a booster.
The cosco apex is another one you could consider as it goes to a highter weight and height and isn't as much as the Britax. It requires you have headrests behind the head of the restraint though, so It won't work if you lack headrests.